If she’s pregnant you’ll soon know, buns nest very closely upon giving birth. Most bun moms are excellent mothers so you don’t have to do much more than provide a nest box and nesting material, plus extra food for Oreo.
It is however very common that unspayed girl buns get false pregnancies. It can be triggered by a whole host of environmental stimuli, rabbits are induced ovulators so they don’t have heat cycles likes dogs do. The bun acts as if it were pregnant, she pulls her own fur (pregnancy hormones make the fur loosen so it’s not self-mutilation), and tries to build a nest. Then she doesnt have baby and the false pregnancy resolves of itself.
You can provider her with a nest box, whether she actually is pregnant or just pseudo-pregnant. It helps her feel safer either way. If she has actual kits, the nest box should be made so that the kits can’t accidentally migrate out of it and get cold. The walls shouldn’t be so high that the mama bun cant jump into the box for feeding time. Mama bun won’t lie in the nest with the kits, she’ll just go there to feed them 1-2 times per day. That’s how rabbits go about motherhood, so that’s how it should be.